The guayule plant, Parthenium argentatum Gray, which grows principally in the southwest United States and Mexico, is known as a potential domestic source of rubber. This plant also provides organic resinous material which is usually a by-product of rubber extraction. The term "guayule resin" is used herein to identify those constituents of the guayule plant (and similar rubber-containing plants) obtained when the plant itself, or resinous rubber prepared from it, is extracted with an oxygenated organic solvent such as acetone, ethyl alcohol and the like. This resinous fraction usually constitutes about 6-9% of the dry weight of the plant before processing. Material obtained by hydrocarbon solvent extraction of aged guayule shrub is also considered resin since it probably contains resin and resin-like low molecular weight rubber.
Since large amounts of rubber will probably be recovered from guayule and guayule-like plants in the future, it is clear that large amounts of guayule resin will become available. Therefore, uses for it and processes for converting it to useful materials will be of increasing interest.
One use for the resin and products made from it is as tackifiers and green strength improvers for rubber compositions. Tack, or more specifically building tack, is a desirable property of rubber. Simply stated, it is a measure of the ability of two unvulcanized rubbers to stick together during fabrication and prior to vulcanization. Related to tack is a rubber's uncured strength, usually called its "green strength." This property is also an important factor during manufacture of rubber articles such as hoses, tires, and belts. Green strength is a measure of an uncured rubber's ability to retain its shape prior to vulcanization (that is, to resist sag). Thus, it can be seen that both tack and green strength are significant properties in fabricating and handling unvulcanized rubber materials.
While natural rubbers (e.g., Hevea), in some instances, possesses sufficient inherent tack and green strength, it is often found that agents are desirably added to them to increase these properties. Synthetic rubbers often lack tack and/or green strength and it is common practice to add agents to them to correct such deficiencies (see, for example, U.S. Pat. No. 4,287,104 to Dimitri and Force).